The present invention is directed, in general, to electronic circuits and, more specifically, to a system and method of increasing a self-resonant frequency of a tuning circuit and an oscillator employing the same.
Harmonic-based oscillators are often employed in system components associated with, for instance, radio frequency (RF) systems wherein phase noise is of concern. The oscillator topology generally incorporates a sustaining amplifier and a tuning circuit. The sustaining amplifier provides sufficient energy during each cycle of operation of the oscillator to make up for any losses associated with resonant structure of the tuning circuit. The tuning circuit generally determines the frequency of oscillation of the oscillator.
Proper operation of an oscillator operating at radio frequencies is typically more difficult to achieve than for oscillators operating at lower frequencies. At radio frequencies, the available gain of the amplifiers is usually somewhat limited and the losses due to parasitic elements associated therewith may become significant. In general, low loss radio frequency capacitors and moderate gain radio frequency amplifiers may be constructed in silicon substrate integrated circuits using conventional fabrication processes designed for radio frequency analog circuits. Technology issues, however, limit the availability of very low loss, radio frequency silicon substrate integrated inductors unless more complex fabrication processes are used.
In some applications, this situation may be avoided by incorporating an external inductive component with the integrated circuit. A radio frequency low loss inductor may be located proximate an integrated circuit on a hybrid assembly containing the integrated circuit or adjacent to the integrated circuit on a printed wiring board. The use of an external inductor typically adds parasitic capacitance and stray inductance that usually degrades the performance of a radio frequency oscillator. These unwanted elements create energy losses that negatively affect the phase noise stability and general operation of the oscillator. See, for instance, xe2x80x9cPhase Noise to Carrier Ratio in LC Oscillators,xe2x80x9d by Qiuting Huang, IEEE Transactions on Circuits and Systems: Fundamental Theory and Applications, Vol. 47, No. 7, pps. 965-980, July 2000, which is incorporated by reference, for a more detailed explanation of the phenomenon.
An additional potential source of loss associated with an oscillator involves the self-resonant frequency of the tuned circuit elements that determine the frequency of oscillation. The impedance of a tuned circuit may be expressed as having both real and imaginary parts. The magnitude of both the real and imaginary parts are frequency dependent. The real part increases from a direct current (DC) resistance value at low frequency to a maximum value at its self-resonant frequency.
In order to maintain a stable oscillation condition, the active part of an oscillator should be capable of adjusting to the magnitude of the real part of this impedance at the frequency of oscillation. If this self-resonant frequency is too close to the frequency of oscillation, oscillator instability may easily occur. These conditions offer design challenges for radio frequency oscillators operating in the one to ten Gigahertz (GHz) range. Therefore, it is of great importance from both a cost and performance perspective to reduce the losses associated with an inductor for use in a radio frequency oscillator.
Accordingly, what is needed in the art is a way to reduce the losses associated with an inductor for use in, for instance, radio frequency applications.
To address the above-discussed deficiencies of the prior art, the present invention provides a segmented inductor for use with a tuning circuit, a method of increasing a self-resonant frequency of a tuning circuit and an oscillator employing the circuit and method.
In one aspect of the present invention, the segmented inductor circuit includes a first inductive element and a second inductive element coupled in series with the first inductive element. An inductance of the first and second inductive elements is substantially equally to a total inductance of the tuning circuit. Additionally, the first and second inductive elements cooperate to reduce a total shunt capacitance associated therewith.
In another aspect of the present invention, the tuning circuit includes a first inductive element having a first intrinsic shunt capacitance. The tuning circuit also includes a second inductive element, coupled in series with the first inductive element, having a second intrinsic shunt capacitance. The first and second inductive elements cooperate to reduce a total shunt capacitance associated therewith thereby increasing a self-resonant frequency of the tuning circuit.
The present invention advantageously introduces the pervasive concept of employing an inductive element in a tuning circuit that has been judiciously segmented. The judicious segmentation of the inductive element allows the summation of the series-coupled shunt capacitances associated with each of the segmented inductors to be smaller than the shunt capacitance associated with a single inductive element. A desired inductance value may be obtained by segmenting the inductor into two or more segments whose series combination provides the advantageous inductance value. The use of a segmented inductor provides a smaller value of the real part of an impedance associated with the inductance at a given frequency. This characteristic thereby reduces the sensitivity of the real part of the impedance to other circuit element variations and generally increases the overall operating xe2x80x9cQxe2x80x9d of the circuit employing the segmented inductor element.
In one embodiment of the present invention, the first and second inductive elements have first and second intrinsic resistances, respectively. The value of the intrinsic resistance of an inductive element is typically proportional to the size and length of the conductor forming the inductive element. In an embodiment to be illustrated and described, the first and second inductive elements are substantially equal in size. In a related, but alternative embodiment, the total shunt capacitance is approximately half a first intrinsic shunt capacitance or a second intrinsic shunt capacitance associated with the first and second inductive elements, respectively.
In one aspect of the present invention, the segmented inductor circuit includes a first inductive element and a second inductive element coupled in series with the first inductive element. An inductance of the first and second inductive elements is substantially equal to a total inductance of the tuning circuit. Additionally, the first and second inductive elements cooperate to reduce a total shunt capacitance associated therewith.
In one embodiment of the present invention, the first and second inductive elements are discrete. In a related, but alternative embodiment, the first and second inductive elements are embodied in an integrated circuit. Of course, the first and second inductive elements may be a hybrid combination involving discrete and integrated circuit components.
In one embodiment of the present invention, the tuning circuit is employable with an oscillator. In a related, but alternative embodiment, the oscillator is embodied in an integrated circuit. Alternatively, the tuning circuit may be employed with other circuits requiring frequency selectivity.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.